2.11.1 Processing Variables and Performance Indicators
In operating food processing equipment, an operator is often concerned with a variety of different process variables. For example, in a heating system, the temperature of a product may require careful monitoring. When milk is pasteurized using a heat exchanger, the temperature of the milk must reach 71°C and be held there for 16 seconds to destroy harmful pathogens. Therefore, the operator must ensure that the temperature reaches the desired value for the specified time or the milk will be either underprocessed (resulting in an unsafe product) or overprocessed with impairment to quality. Similarly, in operating different processing equipment, the flow rate, level, pressure, or weight may be an important variable that requires careful monitoring and control.
Controlled variables are simply those variables that can be controlled in a system. For example, the steam composition, steam flow rate, temperature of a water stream, and level of water in a tank are all variables that can be controlled. When heating milk, temperature is a controlled variable. Other examples of a controlled variable include pressure, density, moisture content, and measurable quality attributes such as color.
Uncontrolled variables are those variables that cannot be controlled when a process is carried out. For example, during operation of an extruder, the impact of operation on the extruder screw surface is an uncontrolled variable.
Manipulated variables are dependent variables that can be changed to bring about a desired outcome. For example, by changing the flow rate of steam to a tank of water, the water temperature will change. This variable may be manipulated by either a human operator or a control mechanism. When heating water in a chamber, the feed rate of water is a manipulated variable. A measured variable is used to alter the manipulated variable. Examples of measured variables are temperature, pH, or pressure, whereas the manipulated variable is the flow rate of a certain material or energy (such as electricity and steam).
Disturbances in the variables are those changes that are not caused by an operator or a control mechanism but result from some change outside the boundaries of the system. Disturbances cause undesirable changes in the output of a system. For example, the temperature of water in a tank is a controlled variable that may be influenced by the inlet flow rate, temperature of the inlet flow, and exit flow rate of water.
Robustness describes how tolerant the system is to changes in process parameters. When the robustness of a control system decreases, a small change in a process parameter makes the system unstable.
Performance communicates the effectiveness of the control system. There is a tradeoff between robustness and performance.
